Many portable electronic or electrical systems, such as personal computers, are powered by rechargeable batteries. Users generally prefer rechargeable batteries, such as Nickel Cadmium (NiCd) batteries, because of their convenience and long life. A significant difficulty encountered in connection with battery-powered devices is predicting the point at which battery capacity is nearly exhausted so that a user may be warned of impending power failure and recharge the battery (or batteries). To be effective, the power failure warning must occur early enough to enable the user to wind up present operations, but not so early as to prevent the user from taking full advantage of remaining battery capacity. While an excessively premature warning will annoy many users, a deferred warning may prevent them from performing orderly power down procedures, resulting in a disastrous loss of work.
Most conventional low battery capacity (or "end point") warning systems employ precisely adjusted voltage comparators that must be individually calibrated by the manufacturer during manufacture and again during service. Such warning systems monitor the absolute, or terminal, voltage of the battery and issue a user warning when the voltage falls below a preset voltage threshold.
The above systems have certain deficiencies. First, the actual operating voltage of batteries will vary among different manufacturers and among different lots of the same manufacturer. In the case of rechargeable batteries, the variance can be greater than the difference in voltage between a battery with half its life remaining and a battery with only a few minutes of life remaining. Further, the terminal voltage of a battery will vary greatly with variations in load, temperature and type of battery. This makes the results of conventional warning systems extremely unreliable in the face of a varying load conditions, such as a portable computer, because it is impossible to determine an exact threshold voltage below which the battery is nearing its end point. As a result, the predetermined voltage threshold must either be set very conservatively, in which case a warning will issue too far in advance when, for example, the load is initially heavy and subsequently lessens, or not conservatively enough, in which case no warning will issue at all when, for example, the load is initially light and suddenly increases. In any case, to achieve results that are even reasonably reliable, the voltage comparators must be manually calibrated by the manufacturer at the time of manufacture and again each time the unit is serviced.
Accordingly, what is needed is a method and apparatus for predicting the end of useful battery life without relying on absolute voltage or requiring precise calibration.